1. Field of the Invention.
The present invention relates to a diffuse reflective article having a nonwoven sheet diffuse reflector containing pores of high light scattering efficiency and having a high photopic reflectance of visible light.
2. Description of Related Art.
Special light reflectant surfaces are used in a variety of applications requiring visible light to be almost completely reflected while providing an even distribution of light from the surface. While mirrored surfaces can provide nearly perfect reflectivity of visible light, the light energy exiting these surfaces does so only at an angle equal to the incident angle. For many applications it is important that visible light be reflected from a surface in a distribution. This property is referred to as diffuse or Lambertian reflectance. Lambertian reflection of light is the uniform diffuse reflection of light from a material in all directions with no directional dependence for the viewer according to Lambert's cosine law. Diffuse reflection originates from a combination of external scattering of light from features on the surface of a material, and internal scattering of light from features within a material. Internal light scattering can arise, for example, from features within a material such as pores, particles or different crystalline phases. The light scattering cross section per unit feature volume of materials containing closely spaced refractive index inhomogenaities is maximized when the mean diameter of the features is slightly less than one-half the wavelength of the incident light. The degree of light scattering is also increased when there is a large difference in the refractive index of the scattering feature and the phase in which it is dispersed.
Diffuse reflectivity of visible light is critical in many applications. Direct view displays used in electronic equipment (e.g., instrument panels, portable computer screens, liquid crystal displays (LCDs)), whether relying on supplemental lights (e.g., backlight) or ambient light, require diffuse reflectant back surfaces to maximize image quality and intensity. Reflectivity is particularly critical with backlit direct view displays in battery powered equipment, where reflectivity improvements directly relate to smaller required light sources and thus lower power demands.
Portable computer LCDs are a substantial and demanding market requiring high levels of diffuse reflection of visible light from very thin materials. For certain markets it is critical that the backlight reflector is relatively thin, i.e., less than 250 μm and often less than 150 μm, to minimize the thickness of the completed display.
The reflective material used in LCD backlights has a significant effect on the brightness, uniformity, color and stability of the backlight unit and, ultimately, the LCD module. For a direct view LCD backlight, requirements for the reflector include high photopic reflectance (e.g., >95%) and high stability under use conditions including cavity temperatures of 50° C. to 70° C., high stability to ultraviolet (UV) light from cold cathode fluorescent lamp (CCFL) sources, high humidity and temperature cycling. In direct view backlights, the reflector is an integral part of the backlight unit and, therefore, the physical properties of the material are also important. Requirements for an edgelit backlight differ in that the operating temperature is typically lower and the need for UV stability is less due to the UV absorption in the light guide. However, additional requirements on edgelit backlight reflectors include the need to make uniform contact with the light guide without damaging it, and minimizing reflector thickness.
Due to the many different applications that exist for reflectant materials, it is not surprising that there are a wide variety of commercially available products with an array of diffuse reflective properties. Until the present invention, the best known diffuse reflective material with excellent diffuse reflectivity was that described in U.S. Pat. No. 4,912,720 and sold under the trademark SPECTRALON® by Labsphere, Inc., North Sutton, N.H., USA. This material comprises lightly packed granules of polytetrafluoroethylene having a void volume of about 30 to 50% and is sintered into a relatively hard cohesive block so as to maintain such void volume. Using the techniques taught by U.S. Pat. No. 4,912,720, it is asserted that exceptionally high diffuse visible light reflectance characteristics can be achieved with this material, with photopic reflectance over the visible wavelengths of light of better than 99%. Despite the advantages of the SPECTRALON material, it is not generally available in very thin films of less than 250 μm, such as those needed for the laptop LCD market, and furthermore at these thickness levels, adequate reflection performance is not obtained.
Gore™ DRP®, produced by W. L. Gore & Associates, Inc., DE, USA, is a reflectant material of expanded polytetrafluoroethylene (PTFE) comprising polymeric nodes interconnected by fibrils defining a microporous structure. This material is highly flexible and has excellent diffuse reflectant properties. Its shortcoming is higher cost. Moreover, at a thickness desirable for many optical display applications (i.e., less than 250 μm), this material has reduced reflectivity over the blue range of the visible spectrum (as shown in comparative example 4 of U.S. Pat. No. 5,976,686). Such a reflector sheet having reduced reflectivity over the blue range requires display manufacturers to modify the display in order to transmit more light in that region in the direction of the viewer, which undesirably consumes more energy.
U.S. Pat. No. 5,976,686 discloses a light conduit containing a 150 μm to 250 μm thick nonwoven polyethylene fabric diffuse light reflector. However, such materials were reported to have an average reflectance varying from 77% to 85%, depending on the thickness, over the wavelength range of 380 to 720 nm. This patent disparages both the random fiber construction of the nonwoven and its variation in thickness as being adversely noticeable in this application and discloses these reflectors in comparative examples.
Filled microvoided poly(ethylene terephthalate) (PET) films, also referred to in this field as “White PET”, are commercial diffuse reflectors used in optical display applications. These materials are sold in different thickness with reflectivity varying with thickness. White PET films around 190 μm thick find utility in notebook personal computer (PC) LCDs and desktop PC LCDs. These films typically have an average reflectance in the visible light wavelengths of about 95%. A 190 μm thick White PET reflector is sold by Toray Industries, Inc. of Chiba, Japan, commercially available as “E60L”. However, E60L suffers from poor resistance to UV radiation and requires a UV coating which raises the cost of the reflector as well as causes a reduction in the reflectivity at wavelengths in the blue region of the visible spectrum (i.e., wavelengths less than about 400 nm).
Improved and inexpensive diffuse reflectors are needed for visible light management applications that will allow for production of more affordable and energy efficient optical displays.